Photorefractive keratectomy (PRK) with pulsed ultraviolet or pulsed infrared laser has limited accuracy, in part, because of pulse-to-pulse power fluctuation of as much as 8%. As disclosed in the prior art patent to L""Esperance, U.S. Pat. No. 4,669,466, there is shown the usage of corneal topography to guide the ablation process. Specifically, a videokeratographic topography apparatus is used to determine the physical, three-dimensional shape of the ablation and this is compared to an ideal ablation profile.
Telfair in U.S. Pat. No. 4,911,711 described the use of a beam profiler means to monitor the homogeneity of the energy profile of individual pulses. In addition, like L""Esperance, Telfair taught the use of intraoperative corneal topography (xe2x80x9csurface diagnosticsxe2x80x9d) to help guide the ablation process. Later, King in U.S. Pat. No., 5,395,356 taught the use of xe2x80x9ca measuring device is included within the apparatus for measuring a parameter which is a function of the corneal surface, such as refractive power or surface curvature.xe2x80x9d
In fact, since it has been difficult to accurately measure the ablation profile on the cornea during or at the end of the treatment, there has been no way to ensure that the desired ablation profile has been achieved in the individual case until many days after treatment. This allows for unwanted overcorrections and undercorrections, requiring retreatment.
In addition to variable pulse energy (fluence), another factor that interferes with PRK accuracy is individual variation in etch rate (amount of tissue removed per pulse for a given energy level). That is to say, that the corneas of some patients ablate slower or faster than the average. Prior art has demonstrated that dry corneal tissue ablates faster than xe2x80x9cwetxe2x80x9d corneal tissue. It is believed that water absorbs the laser energy making it less effective in the removal of corneal tissue. Prior art attempts to measure the actual ablation depth achieved in the individual patient have relied upon corneal topography-type technology and ultrasonic pachymetry, both of which have been very inaccurate and difficult to perform intraoperatively.
It is the principle object of this invention to provide method and means for furnishing extreme accuracy in the ablation profile during performance of the PRK procedure through the use of an energy or power metering means located within the optical path of the laser beam to furnish an actual assessment of the laser energy delivered with each pulse, allowing creation of an energy map used as a reference to indicate the amount and degree of ablation occurring during actual performance of the PRK procedure.
The present invention monitors intraoperative laser energy of each pulse and records the total energy delivered at each point within the ablated area on the cornea. It allows for construction of topography maps of the ablation area based upon energy delivered, rather than actual measurement of the surface of the ablation as taught in prior art. It can be used to prevent overcorrections and undercorrections, reducing potential patient dissatisfaction. Moreover, the present invention adjusts the etch rate (ablation rate) to take into account individual variation in corneal hydration, using the etch rate (ablation rate) to more accurately interconvert the energy map and the physical topography map.
A UV power meter is placed in the optical path of the laser beam. In a preferred embodiment, the power meter is placed distal to the last optical element so that any optical degradation that affects laser performance is taken into account. (FIG. 1). The power meter can be obtained from Coherent, Inc. located in Auburn, Calif., 95602. This meter consists of a UV-B cube and a pulnix camera with a software package developed by Coherent. It can be used in the present invention to monitor the fluence of each laser pulse. Unlike prior art which uses a power meter to profile the energy homogeneity, this power meter is used to size each pulse and to quantify the energy in each pulse.
It is, therefore, an object of this invention to provide sensing means to measure intraoperative pulse-to-pulse energy during photorefractive keratectomy (PRK), using said data in conjunction with the location of the pulse within the ablation zone to determine the cumulative energy thus being achieved, and adjusting said laser to treat more or less at each point based upon the difference between the ideal cumulative energy map and the observed cumulative energy map derived from intraoperative power determination.
Still another object of this invention is to provide means for placement of the sensing means previously referred to, by spatially locating the sensing means distal in the optic train of the laser system.
Still another object of this invention is to provide for the automatic termination of additional laser treatment to all or part of the ablation zone through usage of a sensing means that provides an analysis of the difference between the theoretic cumulative laser energy targeted and the observed cumulative energy map derived from the intraoperative power determination.
Still another object of this invention is to provide automatic delivery of additional laser pulses to all or part of the ablation zone through usage of the sensing means of this invention.
Yet another object of this invention is to provide for adjustment of the etch rate that takes into account the effects of corneal hydration during photorefractive keratectomy.
Still another object of this invention is to provide for an assessment of the corneal hydration through the use of central corneal pachymetry used during performance of the PRK process.
Yet another object of this invention is to provide intraoperative determination of ablation depth.
Yet another object of this invention is to provide for the use of individualized etch rate to compensate for the effects of corneal hydration for use for interconverting the energy map derived from the sensing means to derive a physical topography map of the cornea during performance of the PRK process.